An example of apparatus of this type is described in Canadian Patent CA 1 123 792 which relates more precisely to a mercury seal intended to be fitted on a cylindrical opening of an enclosure so as to close off the enclosure hermetically and, in particular, so as to prevent the gases respectively inside and outside the enclosure from mixing together.
In that prior art apparatus, the annular mercury seal is confined between two elastomer O-rings which keep the mercury seal in place.
Although quite appropriate when the piston is permanently fixed in the cylinder, this solution suffers from reliability problems when the piston is to slide in the cylinder, since wear and stress on the O-ring seals may cause leakage of the sealing liquid (mercury in this case).
In this context, an aim of the present invention is to provide a sealing apparatus which is more reliable than the apparatus of the above-mentioned patent, and which, in particular, may be used quite safely when the pressure difference between the first and second volumes changes and/or is relatively large.